Laser Direct Writing of MnO2/Carbonized Carboxymethylcellulose-Based Composite as High-Performance Electrodes for Supercapacitors

Manganese dioxide and its derivatives are widely used as promising electrode materials for supercapacitors. To achieve the environmentally friendly, simple, and effective material synthesis requirements, the laser direct writing method is utilized to pyrolyze the MnCO3/carboxymethylcellulose (CMC) precursors to MnO2/carbonized CMC (LP-MnO2/CCMC) in a one-step and mask-free way successfully. Here, CMC is utilized as the combustion-supporting agent to promote the conversion of MnCO3 into MnO2. The selected materials have the following advantages: (1) MnCO3 is soluble and can be converted into MnO2 with the promotion of a combustion-supporting agent. (2) CMC is an eco-friendly and soluble carbonaceous material, which is widely used as the precursor and combustion-supporting agent; (3) the redundant part of the MnCO3/CMC precursor can be removed by deionized water, which is simple and convenient. The different mass ratios of MnCO3 and CMC-induced LP-MnO2/CCMC(R1) and LP-MnO2/CCMC(R1/5) composites are investigated in the electrochemical performance toward electrodes, respectively. The LP-MnO2/CCMC(R1/5)-based electrode showed the high specific capacitance of 74.2 F/g (at the current density of 0.1 A/g) and good electrical durability for 1000 times charging–discharging cycles. Simultaneously, the sandwich-like supercapacitor which was assembled by LP-MnO2/CCMC(R1/5) electrodes presents the maximum specific capacitance of 49.7 F/g at the current density of 0.1 A/g. Moreover, the LP-MnO2/CCMC(R1/5)-based energy supply system is used to light a light-emitting diode, which demonstrates the great potential of LP-MnO2/CCMC(R1/5)-based supercapacitors for power devices.


INTRODUCTION
With the increase number of fast-charging mobile electronics and battery-powered vehicles, the high-powered energy storage tools were in tremendous demands. Supercapacitors have gained widespread attention due to their unique characteristics like high specific capacitance, high power density, and long cycle life. 1−4 According to the energy storage mechanism, supercapacitors can be divided into two types: electrical double layered capacitor (EDLC) and pseudocapacitor. As for EDLCs, in the charging process, the polarized electrodes attracted the ions from the electrolyte solution to generate the electric double layers quickly, which led to the charging/discharging cycles in a short time. While for pseudocapacitors, the working mechanism could be ascribed to the Faradaic reactions occurring on the surface of conducting polymers and metal oxide based electrodes, causing the significant change of capacitance. 5 There were a great deal of research studies that focused on these two types of supercapacitors, especially for the electrodes. The common electrode materials contained carbon-based nanomaterials, metal oxides, conducting polymers, and their nanocomposites, such as porous carbon, 6 carbon nanotubes, 7 graphene, 8 MnO x , 9,10 NiO, 11 polyethylene dioxythiophene, 12 and polyaniline, 13 along with some novel materials like metal−organic frameworks, 14 MXenes, 15 metal nitrides, and so on. 16−18 Among them, due to the high theoretical capacity and excellent electrochemical characterizations, the MnO 2 and its derivatives were widespread used as a promising electrode material for supercapacitors. 19 The redox reaction of MnO 4 − or Mn 2+ was universally employed to synthesize the MnO 2 as well as its derivatives, and there are several methods to prepare MnO 2 -based electrodes, for instance, sol−gel processing, electrochemical deposition, coating, and inject-printing method. 20−22 Typically, Chandra and coauthors demonstrated the MnO 2 nanorod-based electrodes by the coating and magnetron sputtering procedures on silver porous-like substrate, which exhibited the high specific capacitance up to ∼796 F/g. 23 ten Elshof and coworkers reported the δ-MnO 2 nanosheets for a flexible micro-supercapacitor via the inject-printing method. The device delivered the volumetric capacitance of 2.4 F·cm −3 and high energy density of 1.8 × 10 −4 Wh/cm 3 . 24 Wang and coauthors presented the asymmetric supercapacitor with MnO 2 -based electrodes and Na 2 SO 4 aqueous electrolyte because the MnO 2 nanoparticles were generated by electrochemical deposition of KMnO 4 in N-dimethylformamid solution. 25 Furthermore, to enhance the electrochemical performance of MnO 2 , many efforts had developed not only on novel structures of MnO 2 but also for the synergetic effect of MnO 2 and the other conductive materials, such as activated carbon, 26 hierarchical porous carbon, 27 graphene, 28−30 and nanotubes. 31,32 Zhao and coworkers reported the MnO 2 nanowire/CoAl-based hierarchical nanocomposite for highperformance supercapacitor, which displayed the high specific capacitance of 944 F/g (at the current density of 1 A/g), good stability, and excellent long-term cycling life. 33 Li and coworkers developed the MnO 2 nanoflakes/hierarchical porous carbon nanocomposites for supercapacitor electrodes by a two-step redox route. 34 Although the aforementioned methods could fabricate the MnO 2 -based materials toward capacitance electrodes comprehensively, the following disadvantages were nonnegligible: (1) the large quantities usage of environmentally harmful reagents; (2) the complexity of the synthesis processes and long time-consuming, for example, the synthesis temperature of MnO 2 -based composites was up to 200°C for 24 h. 35 To address these shortcomings, an effective, compatible, and compact fabrication method for MnO 2 synthesis was required.
Laser direct writing (LDW) could promote the redox reaction process of materials due to the distinctive photothermal effect. Because the laser beam led to the high temperature atmosphere for material, causing the obvious change of physical−chemical properties of the irradiated partial area. Thus, the LDW method was widely utilized to synthesis or change the characterizations of nanomaterials directly in ambient, gas, or liquid conditions, which was effective, scalable, and patternable. Over the past decade, the LDW-induced carbonaceous material, for instance, the laser induced graphene (LIG), the laser reduced graphene oxide (LRGO), and ligninderived carbon were reported to generate the flexible electrodes or supercapacitors. Feng and coauthors demonstrated the LIG-based transparent supercapacitors in one-step with a high specific capacitance of 8.11 mF/cm 2 and a volume capacitance density of 3.16 F/cm 3 (0.05 mA/cm 2 ). 36 Cai and coworkers showed the LDW-induced LRGO−GO−LRGO interdigitated micro-supercapacitors, exhibiting the high capacitance of 12.5 mF/cm 2 at the scan rate of 10 mV/s. 37 Furthermore, LDW was also utilized to investigate the metallic- based materials for electrode synthesis. For instance, Lee et al. designed a flexible micro-supercapacitors with self-generate silver layers by laser-induced growth-sintering technique. 38 Zhu et al. demonstrated LIG-decorated MONPs (M = Ti, Ni, Sn) electrodes by laser ablation in liquid phase conditions. 39 Zhang et al. used the island-bridge structure to enhance the stretchability of micro-supercapacitor arrays and employed the LIG foam to generate a self-powered wireless wearable sensing platform. 2,3 Cheng and coauthors demonstrated a new strategy to fabricate the functional circuits on 3D freeform surfaces by intense pulsed light-induced mass transfer of zinc nanoparticles. 40 However, to our best knowledge, the study of the metallic-based supercapacitor generated by the LDW method was still in small quantity, particularly for MnO 2 -based composites due to the uncompleted redox reaction process and uncertainties of the secondary products.
Here, we used the LDW method to pyrolyze the MnCO 3 and carboxymethylcellulose (CMC)-based precursors for electrode and supercapacitor. CMC was environmentally friendly, which could be easily dissolved in water and showed good material synergy performance with the other soluble materials. The MnCO 3 was also water-soluble and could be oxidized to MnO 2 by laser pyrolyzation process. As for MnCO 3 and CMC composites, CMC was utilized as the combustionsupporting agent to promote the conversion of MnCO 3 into MnO 2 , and the redundant part of the precursor would be removed by deionized (DI) water. The different mass ratios of were also studied for the electrochemical performance, which showed the maximum specific capacitance of 49.7 F/g at the current density of 0.1 A/ g. Moreover, the LP-MnO 2 /CCMC(R1/5)-based energy supply system was used to successfully light a LED, demonstrating the great potential of LP-MnO 2 /CCMC(R1/ 5)-based supercapacitors for power devices.

LDW Synthesis LP-MnO 2 /CCMC Composites.
In this paper, the LDW method was proposed to fabricate LP-MnO 2 /CCMC-based electrodes and supercapacitors. Figure  1a showed the schematic illustration of the LDW system, which mainly contained the continuous wave laser (532 nm), a dichroic filter, and a programmable 3D translation stage. The laser power of 0.3 W and scanning speed of 2.5 mm/s were utilized to prepare LP-MnO 2 /CCMC-based composites, which were further assembled to the sandwich-like supercapacitor with a ZnSO 4 /PVA dielectric layer, as shown in Figure 1b. Figure 1c presented the detailed preparation process of the LP-MnO 2 /CCMC-based electrode.
The morphologies and chemical compositions of LP-MnO 2 / CCMC composites are shown in Figure 2.   Figure S1), the morphology of the LP-MnO 2 /CCMC(R1) composite presented the negligible structure change except for the much higher roughness of the particle surface. Meanwhile, the LP-MnO 2 /CCMC(R1/5) composite showed the significant morphologies changes with the increase of porous and needle-like nanostructures, which could be ascribed to the LDW-induced high throughput release of oxygen-containing gases. Herein, it should be noted that the CMC was employed as a combustion-supporting agent in the LDW process to promote the pyrolyzation of MnCO 3 . When the mass ratio of CMC in MnCO 3 /CMC composites was small (such as MnCO 3 /CMC = 1:1), the combustion-supporting effect could be quite weak and the morphology change of LP-MnO 2 /CCMC(R1) was indistinctive. Figure 2e showed the transmission electron microscopy (TEM) image of LP-MnO 2 /CCMC(R1) composites, which had the large-area of layer shapes. Figure 2f illustrated the TEM image of significant needle-like nanostructures for LP-MnO 2 /CCMC(R1/5) composites. Consecutively, the EDS analysis mappings were utilized to investigate the elements distribution of LP-MnO 2 /CCMC(R1) and LP-MnO 2 /CCMC-(R1/5) composites. As shown in Figure S2, both of the samples contained the C, Mn, Na, and O elements. Distinguishingly, the Na element was derived from the precursor of CMC. Figure S2a−e exhibited the elements mapping of LP-MnO 2 /CCMC(R1). In Figure S2c, the Mn mapping image was mainly concentrated in upper half regions, indicating an uneven distribution of elements, which mainly caused by the uncompleted pyrolyzation of MnCO 3 . Compared with the LP-MnO 2 /CCMC(R1) sample, the LP-MnO 2 /CCMC(R1/5) composites mapping exhibited the more uniform distribution of C, Mn, and O elements, illustrating the good carbonization and pyrolyzation of MnCO 3 to MnO 2 by the LDW method ( Figure S2f−j).
For further investigation study, the chemical compositions of LP-MnO 2 /CCMC(R1) and LP-MnO 2 /CCMC(R1/5) were characterized by the X-ray diffractometer (XRD) test. As shown in Figure 2g, the crystal structure of LP-MnO 2 / CCMC(R1/5) was in high correlation with the face-centered cubic-based MnO 2 , which exhibited three typical peaks at 38.21, 45.26, and 59.82°. Inversely, although the LP-MnO 2 / CCMC(R1) spectrum also had three typical peaks nearby 38.21, 45.26, and 59.82°, the crystal structure of LP-MnO 2 / CCMC(R1) was more similar to the typical peaks of MnCO 3 , which addressed the incomplete pyrolyzation of the MnCO 3 / CMC precursors for the LDW process.
Raman spectroscopy was also adopted to identify the nanostructures and chemical components of LP-MnO 2 / CCMC(R1) and LP-MnO 2 /CCMC(R1/5) composites in Figure 2i. For the Raman spectra of LP-MnO 2 /CCMC(R1) composites, the typical peaks were located in 1387 and 1590 cm −1 , which were attributed to the D-and G-bands. The typical peaks of Raman spectra for LP-MnO 2 /CCMC(R1/5) composites appeared at 1339 and 1587 cm −1 . Noteworthily, the overlapping of the D-and G-bands illustrated the low crystallinity of the carbonaceous flakes. The I D /I G of LP-MnO 2 /CCMC(R1) and LP-MnO 2 /CCMC(R1/5) composites were calculated as 5.515 and 1.642, respectively, which conveyed the much higher crystallinity and less defects of the LP-MnO 2 /CCMC(R1/5) synthesis material. Above the aforementioned results, the MnCO 3 /CMC precursors with a mass ratio of 1:5 were more suitable for preparing the LDWinduced LP-MnO 2 /CCMC(R1/5) electrodes, which would be used in the following sections.

Performance Study of LP-MnO 2 /CCMC-Based Electrodes.
To demonstrate the electrical performance of LP-MnO 2 /CCMC-based electrodes, the two-electrode method was utilized to investigate the characterizations of cyclic voltammetry (CV) and galvanostatic charge−discharge (GCD) tests for LP-MnO 2 /CCMC(R1/5)-based electrodes comprehensively. The potential window of test was set as −0.1 to 0.5 V and the scan speeds were used in 2−100 mV/s. Figure  3a Figure 4a showed the CV curves for potential windows of 0.0−0.5, 0.0−1.0, and 0.0−1.5 V at a scan rate of 10 mV/s. Under different potential windows, the CV curves were all closed in shapes of parallelograms without significant distortions. The further calculated results demonstrated that the LP-MnO 2 /CCMC(R1/5)-based supercapacitors had the largest specific capacitance of 11.77 F/g in the potential window of 0.0−1.0 V. Figure 4b shows the CV curves of LP-MnO 2 /CCMC(R1/5)-based supercapacitors at scan rates of 2−50 mV/s and the potential window of 0.0−1.0 V. It can be found that when the scan rate increased, the areal capacitance of the device increased regularly and the CV curves surrounded parallelograms maintained symmetrically, which illustrated the good capacitive behavior of LP-MnO 2 /CCMC-(R1/5)-based supercapacitors.
The GCD curves for LP-MnO 2 /CCMC(R1/5)-based supercapacitors at current densities of 0.1−2 A/g are recorded in Figure 4c. Figure 4d showed the relationship between specific capacitance and current density of LP-MnO 2 /CCMC-(R1/5)-based supercapacitors. The specific capacitance of the device was getting more and more larger with the continuous decrease of the current density. When the current density was 0.1 A/g, the specific capacitance of the device reached a maximum value of 49.7 F/g. Moreover, according to the calculation equation of energy density (E d ), power density (P d ), and coulombic efficiency (CE), as for LP-MnO 2 /CCMCbased electrodes, the maximum values of the E d , P d , and CE values were ascribed to 26.38 Wh/kg, 1.6 × 10 3 W/kg, and 96.05%, respectively. For the LP-MnO 2 /CCMC-based supercapacitor, the E d , P d , and CE values were calculated as 6.90 Wh/kg, 1.0 × 10 3 W/kg, and 47.83%, respectively. The Ragone plot of LP-MnO 2 /CCMC-based electrodes and supercapacitor with the other reported literature are shown in Figure S4 and Table S3.

Application of LP-MnO 2 /CCMC-Based Supercapacitor.
To further study the energy supply performance of devices, three LP-MnO 2 /CCMC(R1/5)-based supercapacitors were in series connection for application tests. The series connection could increase the device test voltage, and the whole supercapacitor arrays were connected to a Chenhua electrochemical workstation for charging. Herein, the fully LP-MnO 2 /CCMC(R1/5) based energy supply system successfully lighted a LED, as shown in Figure 4e,f, which demonstrated the great potential of LP-MnO 2 /CCMC(R1/5)-based supercapacitors for power devices. Additionally, the electrical performances of the LP-MnO 2 /CCMC-based supercapacitor for stretching and bending tests are shown in Figure S5. In Figure S5a, the current density with bending angles of 3, 5, 7, and 10°is displayed, presenting the specific capacitances of 11.87, 8.97, 5.52, and 2.92 F/g. Figure S5b demonstrated the current density in stretching range of 1.6, 3.8, and 8.9%, which exhibited the minimum specific capacitance of 2.16 F/g. It could be concluded that with the increase of stretching range and bending angle, the specific capacitances of supercapacitor were in general decreasing, respectively.

CONCLUSIONS
In summary, the LP-MnO 2 /CCMC-based electrodes and supercapacitor were fabricated in a one-step and mask-free way by the LDW method. Notably, CMC was utilized as the combustion-supporting agent to promote the conversion of  3 and CMC solution with the mass ratio of 1:1 or 5:1 was prepared as precursors for the LDW process. Herein, the 25 mg/mL MnCO 3 /CMC mixture was poured into a Petri dish and dried at 60°C to obtain the homogeneous thin film, respectively.
The 532 nm continuous wave laser (Verdi G10, Coherent Inc., beam diameter = 20 μm) and the 3D programmable platform (PSA150-11-X, Zolix Inc.) were utilized to pyrolyze the MnCO 3 /CMC thin film for LP-MnO 2 /CCMC(R1) and LP-MnO 2 /CCMC(R1/5) composites in the ambient environment. The laser power and scanning speed were maintained at 0.3 W and 2.5 mm/s in the entire LDW process unless the specific illustration. After the LDW procedures, the generated LP-MnO 2 /CCMC composites were immersed into the DI water for 20 min, which could remove the redundant the MnCO 3 /CMC composites thoroughly. The dried LP-MnO 2 / CCMC composites were grinded into powder and blended with PVDF in the mass ratio of 8:1. Then, dissolving the LP-MnO 2 /CCMC/PVDF mixed powder in N-methylpyrrolidone solution, the compound was uniformly depositing on the carbon paper in 1 cm × 1 cm. After drying the samples at 60°C for 40 min, the LP-MnO 2 /CCMC-based electrodes were finally generated.  As for LP-MnO 2 /CCMC-based electrodes and supercapacitor electrochemical characterizations, the CV tests were performed at scan rates of 2−100 mV/s; the GCD tests were measured at current densities of 0.1−2 A/g; the EIS tests were performed by 0.01−10 5 Hz at open circuit potential.
Moreover, the capacitance (C GCD ) of the supercapacitor at different current densities was obtained by the following eq 1 where I, m, ΔV, and Δt represent the current, the electrode mass, the potential window, and the discharge time, respectively. The device's capacitance (C CV ) at different scan rates was obtained using the following eq 2 where I, m, v, and ΔV ascribe to the current, the electrode mass, the scan rate, and the potential window, respectively.
Energy density (E d ), power density (P d ), and CE of the LP-MnO 2 /CCMC-based electrodes and supercapacitor were calculated by eqs 3−5 as follows where C m and ΔV denote the gravimetric specific capacitance and potential window of supercapacitor, respectively.
where E d and Δt represent the energy density and the discharge time of supercapacitor, respectively.